The present invention relates to an optical switch device which changes the path of signal light used for optical communications or the like.
Optical switch devices are components that are essential to wavelength division multiplexing (WDM) indispensable in an optical network serving as a base of, e.g., the Internet communication network. Optical switch devices of this type include optical waveguide type devices and MEMS (Micro Electro Mechanical System) type devices. Especially, the MEMS type optical switch devices having small movable reflecting surfaces are expected to be promising.
A MEMS-type optical switch device is formed from, e.g., a fixed structure and a reflecting structure having a movable mirror. The fixed structure comprises a substrate serving as a base, an electrode formed on the substrate, and the like. The reflecting structure has a support member and movable member. The movable member which acts as a mirror is separated from the fixed structure and connected to a support member through a spring member such as a torsion spring. Such a structure can be formed using the micromachine technology which implements three-dimensional micromachining by, e.g., performing etching on the basis of thin film formation or photolithography. An optical switch having the above structure performs switching operation of switching an optical path by moving the reflecting structure in accordance with the attracting force or repelling force acting between the fixed structure and the movable reflecting structure.
The above-described optical switch devices that are formed by micromachining are roughly classified into two types. One type is formed by a so-called surface micromachine. The other type is formed by a bulk micromachine.
A device of the former surface micromachine type will be described first. A surface micromachine has an arrangement as shown in FIG. 9. Referring to FIG. 9, support members 902 are pivotally arranged on a substrate 901. A frame 904 is supported by the support members 902 through hinges 903. A mirror 905 is connected to and supported by the frame 904 through a torsion spring (not shown).
Electrode portions 906 which generate an electrostatic force to drive the mirror 905 are formed under the mirror 905 and connected to interconnections (not shown). Such a structure is formed by, e.g., the steps of forming a silicon oxide film on the surface of the substrate, forming the electrode interconnection structure on the substrate, forming a polysilicon film serving as the mirror on the silicon oxide film, and etching a sacrificial film formed from a desired portion of the silicon oxide film using hydrofluoric acid or the like to separate the mirror from the substrate.
The element techniques of the surface micromachine technology are obtained from an application of the process technology for LSI. For this reason, the vertical size of a structure made by forming a thin film is limited to several μm. For an optical switch device in which the distance between the lower electrode portions 906 and the mirror 905 must be set to 10 μm or more to rotate the mirror, the sacrificial film formed from silicon oxide is removed, and simultaneously, the mirror 905 is lifted up by internal stress in the polysilicon film. Alternatively, the support members 902 are pivoted by an electrostatic force to separate the portion of the mirror 905 from the electrode portions 906.
In the bulk micromachine type, an optical switch device is generally formed by individually preparing a substrate that constructs a mirror and a substrate that constructs an electrode and connecting the substrates. Use of an SOI (Silicon On Insulator) substrate has been proposed for mirror formation. A mirror formed using an SOI substrate is formed from not polysilicon that is general for a surface micromachine but single-crystal silicon. In the structure formed from polysilicon, the mirror is warped by stress due to the polycrystal. However, in a mirror made of single-crystal silicon formed by using an SOI substrate, the warp is relatively small.
Manufacture of an optical switch using an SOI substrate will be described below with reference to FIGS. 10A to 10F. First, as shown in FIG. 10A, a trench 1001a is formed on a side (major surface) of an SOI substrate 1001, on which a buried oxide film 1002 is formed, by the known photolithography technique and etching such as DEEP RIE. With this process, a mirror 1004 is formed in a single-crystal silicon layer 1003 on the buried oxide film 1002.
At this time, a metal film such as an Au film is sometimes formed on the surface of the mirror 1004 to increase the reflectance of the mirror 1004. DEEP RIE is a technique for, e.g., dry-etching silicon, in which SF6 gas and C4F8 gas are alternately supplied to repeat etching and sidewall protective film formation so that a trench or hole with an aspect ratio as high as 50 at an etching rate of several μm per min.
Next, a resist pattern having an opening in the formation region of the mirror 1004 is formed on the lower surface of the SOI substrate 1001. The silicon is selectively etched from the lower surface of the SOI substrate 1001 using an etchant such as an aqueous solution of potassium hydroxide. In this etching, the buried oxide film 1002 is used as an etching stopper layer. As shown in FIG. 10B, an opening portion 1001b is formed in the lower surface of the SOI substrate 1001 in correspondence with the formation region of the mirror 1004. Next, a region of the buried oxide film 1002, which is exposed into the opening portion 1001b, is selectively removed using hydrofluoric acid such that the mirror 1004 pivotally supported by the SOI substrate 1001 is formed, as shown in FIG. 10C.
On the other hand, a silicon substrate 1011 is selectively etched by an aqueous solution of potassium hydroxide using a predetermined mask pattern formed from a silicon nitride film or silicon oxide film as a mask. With this process, a recessed structure is formed, as shown in FIG. 10D. Then, a metal film is formed on the recessed structure by deposition or the like. The metal film is patterned by photolithography using known ultra-deep exposure and etching to form an electrode portion 1012, as shown in FIG. 10E.
Finally, the SOI substrate 1001 having the mirror 1004 shown in FIG. 10C and the silicon substrate 1011 shown in FIG. 10E are bonded to manufacture an optical switch device in which the mirror 1004 is moved by applying an electric field, as shown in FIG. 10F.
In manufacturing an optical switch by the above-described surface micromachine, however, a support structure like the support members 902 shown in FIG. 9 is formed as a movable structure in the mirror formation step. For this reason, the yield in the step of forming the support structure is lower than that in the remaining steps. This decreases the manufacturing yield of optical switch devices. In addition, since the presence of movable portions other than the mirror increases the number of movable portions, the reliability of the optical switch decreases.
Manufacturing an optical switch by bulk micromachine includes no sacrificial layer etching step for ensuring the mirror moving space, unlike the above-described manufacturing method using surface micromachine and is therefore advantageous in yield and reliability. However, the manufacturing method shown in FIGS. 10A to 10F has the following problems because the mirror moving space is mainly formed by anisotropic etching of Si using KOH solution or the like. First, to make the mirror pivotal on the SOI substrate on the mirror side, Si must be etched to a depth corresponding to almost the thickness of the substrate. At this time, the thickness of Si to be etched is at least several hundred μm.
When the lower surface of, e.g., a commercially available 6-inch SOI substrate having an Si (100) surface and a thickness of 625 μm is anisotropically etched using an alkali solution, e.g., KOH solution as an etchant, as described above, the substrate is etched and exposes the (111) surface having a tilt angle of about 55°. For example, assume that the thickness of the silicon layer on the buried oxide film is 10 μm, and the thickness of the buried oxide film is 1 μm, the thickness to be Si-etched, as shown in FIG. 10B, is 614 (=625−10−1) μm.
To ensure a 500-μm square mirror region after such Si-etching, a region having an area of about 600-μm square is removed by etching on the lower surface of the SOI substrate. Hence, to form one mirror, a large area that is not related to movement of the mirror is wastefully required. This increases the occupation area of the mirror formation portion in the chip, resulting in disadvantage in increasing the degree of integration of an optical switch device.
Additionally, in this processing method, alignment is necessary on both the upper and lower surfaces of the substrate. A complex step such as a so-called double sided aligner step (double sided exposure step) also needs to be executed. Furthermore, the substrate on the electrode portion formation side also requires etching in a depth of 10 μm or more by KOH solution to form the mirror moving space. This process is performed by anisotropic etching, like the substrate on the mirror formation side. To form the recessed structure serving as the mirror moving space, a region having an area of 10-μm square or more must be occupied first on the surface of the silicon substrate and patterned. For this reason, the degree of integration cannot be increased on the electrode side, either.
Even when a control circuit such as an IC or LSI formed by a planar process should be integrated with the optical switch device, it is very difficult in the above-described electrode substrate forming method started with anisotropic etching to form an IC or LSI necessary for mirror control on the electrode substrate side in advance or form a multilevel interconnection structure. For this reason, in the above-described manufacturing method, formation of a highly integrated element for control or formation of a complex control system which requires a number of electrode interconnections per mirror can hardly be achieved. In the above-described optical switch manufacturing method, the optical switch structure itself can be made compact. However, since an external control circuit is necessary, a device serving as, e.g., an optical switch device having desired performance becomes bulky.